This invention relates to an apparatus for simultaneously exhausting hot furnace gases, recovering heat from such gases, removing pollutants from such gases, protecting against excessive heat radiation from a furnace, and removing dust laden air from the region around a furnace opening. The apparatus is preferably used in conjunction with a rotary furnace of a type commonly used in industrial applications. Such a furnace typically is rotatably mounted, and has an open hearth exposed to the atmosphere. The gases generated within such a furnace from the burning process contain a significant quantity of pollutants, and the region around the open furnace end is typically exposed to high heat radiation and becomes heavily dust laden when charging or cleaning the furnace. These conditions create a hazardous work area and require special precautions in order to provide an environment which is suitable for workmen.
The problem of removing hot exhaust gases from such a furnace is further complicated by the fact that the furnace is movable, thus preventing a sealed exhaust coupling from being made to the furnace. The problem of removing dust-laden air from the region in front of the open furnace is complicated by the fact that this region must be kept open for access to the interior. For this reason, the dust removal equipment must not interfere with the normal use and operation of the furnace opening.
Although the function of exhausting hot furnace gases and the function of removing heavily dust-laden air from the furnace work area both have the common essential intermediate step of filtering particulate matter and pollutants from the gas, it is not desirable to combine the two functions directly into a single exhaust system. Hot furnace gases are typically exhausted from the furnace at temperatures ranging from 1200.degree. F - 1700.degree. F, while dust-laden air from the region around a furnace opening is at temperatures ranging from 100.degree. - 150.degree. F. If these two temperature-divergent gases were to be mixed together and exhausted the resultant mixture would range from 600.degree. F - 800.degree. F. This temperature range exceeds the capability of filtration systems having fabric filters, which usually have an upper temperature operating limit of about 500.degree. F, which means that the mixture must be cooled before being fed into a fabric filter.
The accepted approach to cooling hot gases is to provide a gas-to-gas heat exchanger of a capacity suitable for handling the temperature and flow rate characteristics of the gas. Since the heat exchanger must be inserted into the flow path of the gas mixture ahead of the filtration system, the heat exchanger must be sized for the combined volumetric flow rate of the gases comprising the mixture, even though the original hot furnace gases are the only component requiring cooling. Thus, the use of a single exhaust system increases the size and cost of the heat exchanger due to the increased volumetric flow rate requirements. Furthermore, since the temperature range of the mixture is lowered, the efficiency of a heat exchanger in recovering hot furnace gas heat is also lowered, and the amount of recovered heat which can be usefully transferred for other industrial applications is reduced.
A preferable solution to the problem, which is provided by the present invention, is to separate the two gas flow components until after the hot gases have been cooled, and then combine them for filtration. This solution offers an additional benefit in that the cooling of a smaller volume of gas at a higher initial temperature can be done more efficiently and the recovered heat can therefore be more useful, either for preheating the input furnace air or for providing supplementary heating to some other industrial process. Gas to gas heat exchangers are readily available in the industrial field for accepting the required volume of gas at a temperature of 1200.degree. F - 1700.degree. F and for reducing the output gas temperature to below 500.degree. F. These heat exchangers transfer the heat from the furnace gases to clean air, and the air at an elevated temperature is useful as a preheated furnace input air supply.
The key to solving the foregoing problem in the manner described lies in the design of a suitable hood for the furnace end opening--one which brings the captured hot furnace gases into the exhaust system, and which also captures the dust-laden air from the region surrounding the furnace opening, but which maintains an isolation between the respective gas/air flow paths until after the gases have been cooled. The present invention accomplishes this result through the apparatus disclosed herein.
Prior art devices have utilized the various approaches to solving the problem of collecting pollutants and gas from such furnaces. For example, U.S. Pat. No. 3,822,872, issued July 9, 1974, discloses a furnace fume collector which is movable with the furnace itself. The apparatus disclosed utilizes telescoping and movable duct work for transporting fumes from the furnace in any of a number of furnace positions. U.S. Pat. No. 3,215,425, issued Nov. 2, 1965, discloses a movable seal which clamps around the furnace collar for purposes of capturing furnace gases emitted from the furnace opening. The seal may be disengaged whenever the furnace is moved from its resting position.